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/* How to do an addition with carry in C */
void g (unsigned long a, unsigned long b, unsigned long c);
void f1 (unsigned long a, unsigned long b, unsigned long c, unsigned long i)
{
a += i;
b += (a < i);
c += (b == 0) && (a < i);
g (a, b, c);
}
#define ADD_LIMB(u,v,c) ((u) += (v), (c) = (u) < (v))
void f2 (unsigned long a, unsigned long b, unsigned long c, unsigned long i)
{
unsigned long carry1, carry2;
ADD_LIMB (a, i, carry1);
ADD_LIMB (b, carry1, carry2);
c += carry2;
g (a, b, c);
}
/* Generated code on x86_64...
*** With GCC 6.3.0 (-O3) ***
For f1:
.cfi_startproc
addq %rcx, %rdi
setc %r8b
xorl %eax, %eax
movzbl %r8b, %ecx
addq %rcx, %rsi
sete %al
andq %r8, %rax
addq %rax, %rdx
jmp g@PLT
.cfi_endproc
0000000000000000 <f1>:
0: 48 01 cf add %rcx,%rdi
3: 41 0f 92 c0 setb %r8b
7: 31 c0 xor %eax,%eax
9: 41 0f b6 c8 movzbl %r8b,%ecx
d: 48 01 ce add %rcx,%rsi
10: 0f 94 c0 sete %al
13: 4c 21 c0 and %r8,%rax
16: 48 01 c2 add %rax,%rdx
19: e9 00 00 00 00 jmpq 1e <f1+0x1e>
For f2:
.cfi_startproc
xorl %eax, %eax
addq %rcx, %rdi
setc %al
addq %rax, %rsi
setc %al
movzbl %al, %eax
addq %rax, %rdx
jmp g@PLT
.cfi_endproc
0000000000000020 <f2>:
20: 31 c0 xor %eax,%eax
22: 48 01 cf add %rcx,%rdi
25: 0f 92 c0 setb %al
28: 48 01 c6 add %rax,%rsi
2b: 0f 92 c0 setb %al
2e: 0f b6 c0 movzbl %al,%eax
31: 48 01 c2 add %rax,%rdx
34: e9 00 00 00 00 jmpq 39 <f2+0x19>
*** With Clang 3.9.1 (-O3) ***
0000000000000000 <f1>:
0: 31 c0 xor %eax,%eax
2: 48 01 cf add %rcx,%rdi
5: 0f 92 c0 setb %al
8: 48 01 c6 add %rax,%rsi
b: 0f 94 c1 sete %cl
e: 20 c8 and %cl,%al
10: 0f b6 c0 movzbl %al,%eax
13: 48 01 c2 add %rax,%rdx
16: e9 00 00 00 00 jmpq 1b <f1+0x1b>
0000000000000020 <f2>:
20: 48 01 cf add %rcx,%rdi
23: 48 19 c0 sbb %rax,%rax
26: 83 e0 01 and $0x1,%eax
29: 48 01 c6 add %rax,%rsi
2c: 48 83 d2 00 adc $0x0,%rdx
30: e9 00 00 00 00 jmpq 35 <f2+0x15>
The f2 solution is better in both cases! But Clang could have done better.
Bug report: https://llvm.org/bugs/show_bug.cgi?id=31755
*/
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